1. Field of the Invention
This disclosure relates to polymer particles that can be utilized in the preparation of press felts and the like for papermaking processes and, more specifically, in the press section of papermaking machinery. More particularly, this disclosure relates to a dispersion that contains both polymer particles and fibers or other solids and that is utilized in press felts (or fabrics).
2. Discussion of Background Information
Papermaking processes can include the steps of forming the paper fibers into a matted sheet, dewatering the sheet, pressing the matted sheet through rollers to continue the dewatering process and to give the consolidated sheet its desired texture, and further drying the sheet as necessary to remove any remaining excess water from the sheet. Consequently, a papermaking machine generally can include three pertinent sections: the forming section, the press section and the dryer section.
The procedure of papermaking can begin in the forming section with preparation of a pulp slurry. The pulp slurry can be carried through the forming section of the papermaking machine on a forming fabric, not unlike a porous conveyor belt, where the pulp slurry can be formed into a sheet. In the forming section, the sheet can be formed and transported to the press section of the papermaking machine where the process of removing the water from the sheet, begun in the forming section, can be continued.
In the press section of a papermaking machine, the wet, matted sheet of paper fibers can be transported on one or more press fabrics and can be passed through rollers along with the press fabrics such that, in a press nip, at least some of the remaining water can be squeezed out of the sheet and can be absorbed through the permeable press fabric. As compression is increased between the rollers, water removal can be likewise increased. The function of pressing also can consolidate the sheet and provide texture to the surface of the sheet.
Some possible beneficial press felt or fabric properties can include resistance to abrasion, resistance to compaction, heat and chemical resistance as well as strength, permeability and caliper retention.
Within the press section, the sheet can be supported and transported via one or more fabrics referred to as “press felts” (or “press fabrics”). For purposes of this disclosure, the term “press felts” (or “press fabrics”) as used herein shall refer to those fabrics that can be used in the press section of a papermaking machine to support and transport the formed sheet of paper fibers to the dryer section of the machine where even more water can be removed.
Press felts can, for example, include a base fabric (for example a woven or non-woven cloth) that can have a staple fiber batt that can be needle punched thereto. In many press felts, multiple layers of batt fibers can be needle punched to the paper side of the base cloth.
The base fabric of press felts can, for example, be made of 100 percent synthetics, primarily nylon polymers, although polyester and other materials can also be utilized.
It will be appreciated that the term “base fabric,” as used herein, refers to the underlying substrate of the press felt and can include scrim and composite structures as well as those woven and non-woven fabrics known in the art as being suitable for use in press felts for papermaking machinery. Base fabrics can be, for example, woven or otherwise constructed with cabled monofilaments, plied multifilaments, spun yarns or single monofilaments. Base fabrics can be utilized in a single layer or multilayer mesh, and can be woven as endless belts or woven flat and joined with seams. The weave of the base fabric can be constructed to affect pressure uniformity, flow resistance, void volume and compression properties. These base fabrics can be classified as conventional (endless) designs, stratified (laminated) designs, and seam fabrics. The monofilaments or fibers used therein can be, for example, round in cross-section, flat monofilaments, and hollow monofilaments as the fibers used in the base fabric. Alternatively, for example, the base fabric can be a scrim, e.g., an extruded netting, or a composite structure, e.g., an extruded spun-bonded sheet, both of these types of substrates.
The batt can be made from any suitable material, such as, and for example, from nylon fibers or other similar synthetic materials, which fibers can be, for example, round in cross section.
It will be appreciated that, for the purposes of this disclosure, the term “batt” refers to essentially any kind of assembly or web of fibers other than the base fabric which can be suitable for use in press felts, and is not necessarily limited to conventional batting. The fibers can be carded into a uniform web to form the batt before being needle punched onto the base fabric, for example in a series of layers. Moreover, the batt fibers can be needle punched into the base fabric with the fibers oriented in the cross machine direction or in the machine direction, although alternative methods for needle punching now exist. The needling process can be engineered to affect the density, surface properties and permeability of the press fabric.
Permeable fabrics such as press felts can be prone to surface wear. This can especially be true when the batt structure of the fabric can be stratified and a finer dtex fiber can be utilized on the surface of the fabric to form a fine “cap” layer, with coarser layers of fiber underneath. It can be difficult to needlepunch this fine “cap” layer into the coarse underlay effectively, to be both strong and wear resistant, as well as keeping the fine fiber on the surface in a homogenous layer to provide sheet support, enhance dewatering, make the sheet smoother, etc. Finer dtex fibers can be inherently weak. Resin treatments and low melt binding of fibers can be utilized to reinforce these weak fine diameter surface fibers. However, the presence of these elements can change the openness, porosity, density and flow properties of the surface of the fabric, which can result in negative effects on performance.
Polymer particles added to a fiber network flow through and along such a fiber network. Resin films, networks and coatings have been contemplated to create specific slightly permeable or impermeable surfaces. Such films, networks and coatings generally exhibit lack of void volume and ineffective pore structure for water handling and sheet release. Resultant coatings typically have a porosity that results from the flow and binding of the resin film to the existing fiber network.
WO 2004/085727, the subject matter of which is incorporated herein in its entirety, discloses an industrial fabric comprising a layer of batt fibers optionally needled to a base cloth, whereby during manufacture of the fabric a dispersion of particulate, polymeric material has been applied to the layer of batt of fibers and thermally activated to provide a discontinuous layer containing a mixture of batt fibers and a polymer-bat fibers matrix.
U.S. Pat. No. 6,712,940, which is incorporated herein by reference, is directed to a press felt.